I can see how to alert when a regex appears in a log file.
I can see how to alert when a regex does not appear on EVERY line of a log file.
But I can't see how to alert when a regex has not appeared in a log file over a period of time.

I could create a bash script which regex's the file for me, and writes out to a file which monit monitors, but would rather keep the solution within monit.

Not sure that this works John. I entered the following:
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BenSep 7 '10 at 21:51

check file system.log with path /var/log/system.log if NOT match "BLAH" within 2 cycles then alert group prices
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BenSep 7 '10 at 21:51

This will alert on every line which does not match the pattern "BLAH". Which is different to what I want to do. I want to alert when I don't receive something over a period of time.
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BenSep 7 '10 at 21:52

If that entry is not working I would then post to the monit mailing list and ask them. Sorry if that didn't help.
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JohnSep 9 '10 at 0:31